UNIVERSITY     OF    CALIFORNIA 
COLLEGE  OF  AGRICULTURE 

BERKELEY 


AGRICULTURAL  EXPERIMENT  STATION 

BENJ.    IDE    WHEELER,    PRESIDENT 
THOMAS    FORSYTH    HUNT,    Dean    and    Director 
H.   E.   VAN    NORMAN,   Vice-Director  and   Dean 
University  Farm   School 


CIRCULAR  No.  139. 

THE   GENERATION    OF   HYDROCYANIC   ACID  GAS  IN 
FUMIGATION  BY  PORTABLE  MACHINES 

By  H.  D.  Young. 

The  latest  important  development  in  fumigation  practice  has  been  the 
use  of  portable  machines  for  generating  hydrocyanic  acid  gas  outside 
of  the  fumigating  tents.  During  the  past  two  years,  the  commercial 
success  of  machine  generation  has  become  an  assured  fact.  This  has 
been  accomplished  chiefly  by  the  Owl  fumigating  machine  (Fig.  1). 

The  new  method  has  a  number  of  advantages  over  the  old  one  of  using 
pots.     In  the  machine  the  cyanide  is  used  dissolved  in  water.     This 


Figure   I.     Owl   Fumigating  Machine.      (Season   1915.) 

solution  can  be  measured  more  accurately  than  lump  cyanide  can  be 
weighed  at  night  under  field  conditions,  so  that  the  dosage  is  made  more 
accurate.  As  the  materials  for  a  number  of  charges  are  all  contained  in 
the  machine,  very  much  less  handling  of  the  chemicals  is  required, 


*Paper   No.    13,   Citrus   Experiment   Station,    College   of   Agriculture,   University   of 
California,  Riverside,  California. 

18682 


involving  some  economy  of  material  and  much  greater  cleanliness.  The 
reaction  also  takes  place  with  great  rapidity  so  that  the  gas  is  produced 
almost  instantaneously;  while  in  the  pots  it  takes  place  much  more 
slowly,  since  the  solid  cyanide  must  first  dissolve.  The  gas  being 
delivered  so  quickly  under  the  tent  should  be  more  effective  than  the 
slowly  generated  gas  from  pots,  as  in  the  latter  case  part  of  the  gas 
will  have  leaked  out  of  the  tent  before  the  generation  is  complete  and 
the  maximum  possible  concentration  will  never  be  obtained. 

In  pot  fumigation  there  is  always  a  greater  or  less  amount  of  injury 
done  to  the  tents  by  the  spilling  of  sulfuric  acid,  dragging  the  tents 
through  the  acid  residue,   etc.     This  not   only  greatly  increases  the 


IB, 


Figure   II.     Cyanofumer.      (Season    1915.) 

expense  of  fumigation,  but  it  impairs  the  effectiveness  of  fumiga- 
tion because  of  the  greater  tent  leakage  caused  by  small  holes  which 
are  overlooked.  Machines  should  almost  entirely  eliminate  tent  burn- 
ing, since  the  acid  is  not  handled  for  each  tree  and  since  the  residue 
can  be  disposed  of  in  a  safe  place. 

Description    of    the    Owl     Fumigating     Machine. 

The  machine  consists  of  a  cylindrical  drum  within  which  there  is  a 
tray  suspended.  Mounted  above  the  drum  are  two  reservoirs  for  sul- 
furic acid  and  cyanide  solution,  respectively.  These  solutions  are 
measured  in  the  cylinders  shown  in  Fig.  I,  and  are  then  run  within  the 
drum.  The  gas  is  very  quickly  generated  and  by  its  own  pressure 
forced  through  the  outlet  hose  under  the  tent. 


—  3  — 

There  is  in  this  machine  no  radical  departure  from  the  dosages  used 
in  pot  fumigation.  The  charges  are  measured  separately  for  each  tree, 
and  in  the  same  proportions  that  would  be  used  in  a  pot.  The  chief 
difference  in  principle  is  that  in  the  machine  the  cyanide  is  added  in 
solution  instead  of  in  the  solid  form  as  in  the  pots,  and  the  generation 
therefore  takes  place  much  more  quickly.  It  has  previously  been 
reported  that  acid  burns  of  the  tents  have  been  almost  entirely  elimi- 
nated by  this  machine.*  Several  cases  of  tent  burning,  however,  have 
been  recently  reported. 

Description    of   the   Cyanofumer. 

A  new  machine  is  being  introduced  this  season,  called  the  Cyanofumer 
(Fig.  II).  It  consists  essentially  of  two  tanks,  one  above  the  other. 
In  the  lower,  tank  is  placed  the  sulfuric  acid  and  water,  in  the  upper 
one,  the  cyanide  solution.  By  the  action  of  a  suitable  pump,  measured 
quantities  of  the  cyanide  solution  are  forced  into  the  tank  containing 
the  acid  and  water,  and  the  gas  is  generated  almost  instantly  and  dis- 
charges through  the  delivery  hose  with  considerable  force.  The  pump 
is  graduated  on  the  basis  of  a  standard  solution  obtained  by  dissolving 
200  pounds  of  pure  sodium  cyanide  in  50  gallons  of  water.  With  a 
solution  of  any  different  strength  it  would  of  course  give  other  amounts 
than  those  marked  on  the  pump  cylinder.  The  basic  principle  involved 
is  that  small  successive  quantities  of  cyanide  solution  are  added  to  a 
large  amount  of  acid  and  water  until  the  acid  is  nearly  exhausted.  This 
constitutes  such  a  radical  departure  from  the  accepted  methods  of 
fumigation  that  its  accuracy  has  been  widely  questioned. 

With  the  machine  charged  to  its  full  capacity,  600  ounces  of  sodium 
cyanide  dissolved  in  1,440  ounces  of  water  are  placed  in  the  upper 
reservoir  b,  Figure  III,  while  600  ounces  of  water  and  600  ounces  of 
concentrated  sulfuric  acid  are  placed  in  the  lower  reservoir,  c,  or 
enough  acid  to  generate  all  of  the  cyanide  solution  at  once,  if  the  two 
were  to  be  mixed.  Assuming  that  the  trees  to  be  fumigated  each  require 
10  ounces  of  sodium  cyanide,  the  pump,  a,  is  set  at  "10,"  the  lever  raised 
to  that  point  and  as  it  is  forced  down  24  ounces  of  solution  containing 
10  ounces  of  cyanide,  if  the  standard  solution  is  used,  are  forced  through 
the  pipe,  /,  into  the  acid  and  water.  Instead  of  the  ratio  being  1  ounce 
of  cyanide,  1J  ounces  of  sulfuric  acid  and  2  ounces  of  water,  as  is  the 
case  in  pot  fumigation,  the  ratio  in  the  first  charge  delivered  is  1  of 
sodium  cyanide,  60  sulfuric  acid,  62.4  water.  As  successive  charges 
are  added  the  acid  becomes  proportionately  less  and  less,  as  it  is  used 
up  by  the  sodium  cyanide;  while  the  amount  of  water  is  continually 
increased,  due  to  that  added,  in  which  the  cyanide  is  dissolved.     The 


*Geo.   P.   Gray,   Monthly  Bulletin,   Vol.   IV,   No.   2,    "New  Fumigating-  Machines." 
Also  Proc.   45,  Fruit  Growers  Convention,   pp.  199-208. 


—  4  — 


ratio  for  the  sixtieth  charge  would,  therefore,  be  1,  27,  204.     At  no 

time  would  the  proportions  be  those  which  have  been  formerly  used. 

In  view  of  the  great  importance  which  has  been  placed  on  the  neces- 


a — Pump. 

b — Cyanide   solution. 

c — Acid   and   water. 

d — Vent   for   discharging  residue. 


e — Gas  outlet. 

f — Pipe  delivering  cyanide  solution  from  pump. 

g — Filling  vent  for  cyanide  tank. 

h — Filling  vent  for  acid  and  water. 


sity  of  accurate  proportions,  prospective  fumigators  are  thoroughly 
justified  in  requesting  an  investigation  of  such  a  radical  departure 
from  the  accepted  standards. 

Drip   From  the   Delivery   Hose. 

There  sometimes  occurs  a  slight  amount  of  liquid  discharge  from  the 
end  of  the  delivery  hose.  This  is  rather  disquieting  to  the  fumigator, 
as  it  suggests  acid  tent  burning.  The  drip  from  over  three  hundred 
charges  was  collected  and  analyzed  without  finding  a  trace  of  free 
sulfuric  acid. 


—  5  — 

Gas  Generation. 

The  following  tables  present  the  results  of  an  investigation  of  the 
accuracy  of  the  new  method.  Samples  of  the  solution  in  the  generating 
machine  were  taken  after  each  five  charges,  and  the  amount  of  hydro- 
cyanic acid  determined.  The  difference  is  assumed  to  be  the  amount  of 
hydrocyanic  acid  gas  delivered  through  the  hose. 

TABLE   I. 
Charges    Generated    at    One- Minute    Intervals. 

Amount  of  HCN  found  in  residue.     Machine  charged  for  fifty  10 -ounce  charges. 


Sample  taken  after  the 

Indicated  number 

of  charges 


Total  amount 

NaCN  added. 

Ounces 


Total  amount 

HCN  found 

in  residue. 

Ounces 


Increase  in 

HCN  due  to 

5  charges. 

Ounces 


Per  cent  HCN 

in  residue 

due  to  5 

charges 


Per  cent  gas 
evolved  by 
difference 


5  charges 
10  charges 
15  charges 
20  charges 
25  charges 
30  charges 
35  charges 
40  charges 
45  charges 
50  charges 


50 

100 
150 
200 
250 
300 
350 
400 
450 
500 


1.17 
3.00 

4.60 
6.12 
8.06 
10,10 
12.74 
15.05 
16.60 
18.63 


1.17 
1.83 
1.60 
1.52 
1.94 
2.04 
2.64 
2.31 
1.55 
2.03 


2.3 
3.6 
3.2 
3.0 
3.8 
4.1 
5.3 
4.5 
3.1 
4.1 


97.3 
96.4 
£6.8 
97.0 
96.2 
95.9 
94.7 
95.5 
96.9 
95.9 


The  preceding  table  shows  the  extremely  high  and  uniform  per  cent 
of  gas  evolved  when  the  machine  is  working  under  the  conditions  to 
which  it  is  best  adapted,  that  is,  fairly  high  charges  at  short  intervals. 
Almost  as  satisfactory  results  are  obtained  with  a  longer  interval  (Table 
II),  but  toward  the  end  of  the  run  the  amount  of  gas  delivered  begins 
to  fall  off  slightly.  A  rate  of  thirty  charges  per  hour  is  probably  as 
slow  as  it  is  wise  to  run  the  machine,  and  with  small  charges,  this  is 
too  slow. 


TABLE  II. 
Charges   Generated    at   Two- Minute    Intervals. 

Amount  of  HCN  found  In  residue.     Machine  charged  for  fifty  10 -ounce  charges. 


Sample  taken  after  the 

indicated  number 

of  charges 

Total  amount 

NaCN  added. 

Ounces 

Total  amount 

HCN  found 

in  residue. 

Ounces 

Increase  in 

HCN  due  to 

5  charges. 

Ounces 

Per  cent  HCN 

in  residue 

due  to  5 

charges 

Per  cent  gas 

evolved  by 
difference 

5  charges .__ 

50 
100 
150 
200 
250 
300 
350 
400 
450 
500 

1.68 

2.83 

4.14 

6.24 

9.03 

12.90 

16.22 

20.15 

23.56 

28.08 

1.68 

1.15 
1.31 

2.10 
2.79 
3.87 
2.32 
3.93 
3.41 
4.52 

3.4 
2.4 
2.5 
4.2 
5.6 
7.7 
6.5 
7.9 
6.8 
9.0 

96.6 

10  charges _. 

97.6 

15  charges  __    __  _    _ 

97.5 

20  charges 

95.8 

25  charges  __ 

94.4 

301  charges    _ 

92.3 

35  charges 

93.5 

40  charges 

92.1 

45  charges 

93.2 

50'  charges 

91.0 

—  6  — 

TABLE   III. 
Charges    Generated    at    One-Minute    Intervals. 

Amount   of   HCN    found   in    residue.      Machine   charged    for    fifty   4-ounce    charges. 


Sample  taken  after  the 

indicated  number 

of  charges 

Total  amount 

NaCN  added. 

Ounces 

Total  amount 

HCN  found 

in  residue. 

Ounces 

Increase  in 

HCN  due  to 

5  charges. 

Ounces 

Per  cent  HCN 
in  residue 
due  to  5 
charges 

Per  cent  gas 
evolved  by 
difference 

5  charges    _    

28 

•    48 

68 

88 

108 

128 

148 

168 

188 

208 

1.92 

2.90 

4.02 

5.06 

6.44 

7.71 

9.36    • 
10.57 
12.01 
13.66 

1.92 
.98 
1.12 
1.02 
1.38 
1.27 
1.65 
1.21 
1.44 
1.65 

6.9 
4.9 
5.5 
5.0 
6.9 
6,3 
8.2 
6.1 
7.1 
8.2 

93,1 

10'  charges 

95.1 

15  charges 

94.5 

201  charges 

95.0 

25  charges __ 

93.1 

30  charges  _  _      

93.7 

35  charges  __  _ 

91.8 

40  charges 

93.9 

45  charges  — 

92.9 

50  charges 

91.8 

Table  III  shows  the  results  obtained  with  four-ounce  charges.  A 
smaller,  but  still  efficient,  amount  of  gas  is  developed.  With  the  smaller 
charges,  the  temperature  of  the  acid  solution  is  lower  (because  of  the 
heat  taken  up  by  the  machine  from  the  smaller  quantity  of  solution 
used),  and  the  amount  of  gas  evolved  consequently  somewhat  less. 

The  effectiveness  of  the  machine  depends  on  maintaining  as  high  a 
temperature  as  possible.  The  gas  is  not  held  in  solution  to  a  great 
extent  by  a  hot  solution,  but  a  cold  one  retains  great  amounts  of  it.  As 
the  temperature  falls,  therefore,  the  amount  of  loss  increases.  The 
necessary  high  temperature  is  obtained  at  the  beginning  by  the  mixing 
of  sulfuric  acid  and  water.  These  should  not  be  placed  in  the  machine 
until  the  last  possible  moment  before  starting  the  series  of  generation. 
By  the  rapid  addition  of  the  alkaline  solution  of  sodium  cyanide  to  the 
hot  acid,  enough  heat  is  formed  to  maintain  a  satisfactory  temperature. 
If  climatic  conditions  should  render  it  difficult  to  keep  the  temperature 
high  enough,  this  might  be  done  by  further  additions  of  sulfuric  acid, 
or  by  jacketing  the  machine  to  lessen  the  heat  loss. 

Comparison  With  Pot  Generation. 

The  amount  of  gas  given  off  by  the  machine  is,  under  such  conditions 
as  given  in  Table  I,  greater  than  the  usual  amount  given  off  by  pots. 
The  latter  give  a  very  irregular  yield  of  gas,  varying  probably  from 
85  per  cent  to  95  per  cent.  The  average  of  a  small  number  of  analyses 
made  in  connection  with  this  investigation  was  92.5  per  cent.  The 
smaller  yield  from  the  pots  probably  is  due  to  a  lower  temperature  than 
that  obtained  in  the  machine.  Using  lump  cyanide  the  reaction  pro- 
ceeds only  as  the  lumps  dissolve  so  that  ten  or  fifteen  minutes  may  be 
required  for  complete  generation.  By  this  time  enough  heat  has  been 
given  off  or  absorbed  by  the  pot  to  lower  the  temperature  of  the  solution 
to  a  point  where  a  larger  amount  of  the  gas  will  be  absorbed.     This 


Figure   IV.     Gasometer  for    collecting   hydrocyanic   acid   gas.      (The   delivery  tube  from   a 
Cyanofumer   may   be  seen.) 


■   "if   BiTm* 

1  JHPr 


Figure   V.     Gasometer   immediately   after   receiving   the    gas   from   a  ten-ounce    charge 

in  the  Cyanofumer. 


—  8  — 

factor  would  vary  greatly  with  the  size  of  the  charge  since  small  quan- 
tities of  solution  would  naturally  lose  proportionately  more  heat  and 
so  retain  more  gas. 

Direct  Determination  of  Gas  Generated. 

The  figures  given  above,  on  the  percentage  of  gas  generated,  are 
based  on  the  "indirect"  method  of  determination,  the  one  which  has 
been  the  basis  of  practically  all  fumigation  schedules.  In  this  method 
the  amount  of  hydrocyanic  acid  left  in  the  residue  is  determined,  and 
the  assumption  is  made  that  the  difference  between  the  per  cent  so 
obtained  and  100  per  cent  represented  the  gas  evolved.  The  error 
involved  in  this  comparison  is,  of  course,  greater  or  less  depending  on 
the  amount  of  decomposition  of  the  gas.  To  determine  the  extent  of  the 
error  with  the  machine,  the  apparatus  shown  in  Figures  IV  and  V  was 
constructed.  It  consists  of  a  metal  tank  24  inches  in  diameter  by 
36  inches  deep,  within  which  a  similar  tank  is  inverted.  The  delivery 
hose  from  the  Cyanofumer  was  connected  with  a  pipe  which  rises 
within  the  tank  nearly  to  its  top.  The  tank  having  been  filled  with 
an  alkali  solution,  and  the  weight  of  the  inner  tank  counterbalanced  to 
prevent  back  pressure  on  the  machine,  the  gas  from  a  ten-ounce  charge 
raised  the  inner  tank  as  shown  in  Figure  V.  After  closing  the  delivery 
pipe,  the  gas  was  slowly  absorbed  by  the  alkaline  solution  in  the  tank. 
Analysis  of  this  solution  showed  an  amount  of  sodium  cyanide  equiva- 
lent to  95.1  per  cent  of  the  theoretical  yield  of  hydrocyanic  acid  gas. 
As  this  was  obtained  under  the  conditions  which  showed  from  2  per  cent 
to  4  per  cent  of  gas  in  residue,  the  amount  of  decomposition  must  be 
very  slight. 

SUMMARY. 

The  principal  advantages  of  machine  fumigation  in  general  are : 

Greater  accuracy  of  dosage,  cleanliness,  rapid  generation  so  that  a 
greater  concentration  of  gas  under  the  tent  is  obtained,  and  a  lessening 
of  tent  burning.  (This  probably  constitutes  the  greatest  saving  of  the 
new  methods.) 

The  Cyanofumer  introduces  an  entirely  different  ratio  in  the  dosage 
schedule,  since  successive  quantities  of  sodium  cyanide  are  added  to  a 
large  amount  of  sulfuric  acid. 

The  amounts  of  sulfuric  acid,  water  and  cyanide  recommended  for 
use,  give  a  high  and  uniform  production  of  gas  under  the  proper  con- 
ditions. 

The  best  production  of  gas  is  obtained  with  a  high  temperature. 

It  is  extremely  important  to  keep  the  cyanide  solution  and  tank 
scrupulously  clean.  Any  dirt  or  small  bits  of  wood  may  interrupt  the 
pump  and  so  make  the  charges  irregular.  Under  normal  conditions 
with  clean  solutions,  the  pump  works  with  great  regularity. 


